This invention relates generally to magnetic structures and more particularly to magnetic structures which are particularly suited for use in nuclear magnetic resonance applications such as for well logging.
Before describing the background of the subject invention it should be pointed out that, while the subject invention is particularly suited for use in NMR systems for well logging, the magnetic structures of this invention can be used in other application as well. Thus there is no intention to limit the generality of the present invention to the field of NMR well logging.
As is known fluid flow properties of porous media have long been of interest in the oil industry. A timur "Pulsed Nuclear Magnetic Residence Studies of Porosity, Movable Fluid, and Permeability of Sandstones," in the Journal of Petroleum Technology, June 1969, page 775, proved experimentally that NMR methods provide a rapid non-destructive determination of porosity, movable fluid, and permeability of rock formation.
It is known that when an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of a static magnetic field is characterized by the parameter, T1, known as the spin-lattice relaxation time.
It has been observed that the mechanism which determines the value of T1 depends on molecular dynamics. In liquids, molecular dynamics are a function of molecular size and inter-molecular interactions. Therefore, water and different types of oil have different T1 values.
In the heterogenous media, such as a porous solid which contains liquid in its pores, the dynamics of the molecules close to the solid surface are also significant and differ from the dynamics of the bulk liquid. It may thus be appreciated that the T1 parameter provides valuable information relating to well logging parameters.
There exist a number of techniques for disturbing the equilibrium of an assembly of magnetic moments, such as those of hydrogen nuclei, for T1 parameter measurements. Each of these techniques provides means for measuring T1 of a rock formation within a certain volume (called the "sensitive volume") which is determined mainly by the shape of the magnetic field surrounding the magnetic structure. The signal-to-noise ratio of the measurement is limited by the ratio of the volume of the sensitive volume to the uniformity (maximum flux density minus minimum flux density) of the magnetic field within said volume, and increases in proportion to this ratio.
In any given nuclear magnetic resonance instrument configuration, the apparatus will respond only to nuclei residing within the sensitive volume. In the present invention and prior art instruments described herein, the boundaries of the sensitive volume are determined by radiation patterns of transmitting and receiving antennae as well as a combination of the detailed structure of the magnetic field with the receiver's frequency passband. The radio frequency that a given nucleus will respond to or emit when excited is proportional to the flux density of the magnetic field in which it is immersed. The proportionality factor depends upon the nuclear species. For hydrogen nuclei, that factor is 42.5759 MHz/Tesla. If the NMR receiver's passband extends from 1.30 MHz to 1.31 MHz, the instrument will be sensitive to hydrogen nuclei in regions of the magnetic field that have flux densities between 30.5 mT and 30.8 mT, providing the antenna radiation pattern allows receiving sufficient signal from that locations. If it is desired to study nuclei located with a particular region, the magnetic field structure, antenna radiation pattern and receiver passband must all be adjusted to be sensitive to that and only that region. Since the signal-to-noise ratio of the resulting signal is proportional to (among other factors) the square root of the receiver passband width, it is important to minimize the variation of the magnetic field within the desired sensitive volume; smaller variations (better field uniformity) mean a better signal-to-noise ratio. Since the signal-to-noise ratio also increases with increasing frequency, the nominal magnetic field intensity within the volume is also very important. It is immaterial whether this nominal intensity is defined as the central value, average value on some other value within the range of values encompassed by the sensitive volume because only large differences in signal-to-noise ratio are significant.
One technique for measuring T1 of a rock formation is exemplified by what is known as the "Schlumberger Nuclear Magnetic Logging Tool". That tool is described by R. C. Herrick, S. H. Couturie, and D. L. Best in "An Improved Nuclear Magnetic Logging System and Its Application to Formation Evaluation", SPE8361 presented at the 54th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, held in Las Vegas, Nev., Sept. 23-26, 1979, and also by R. J. S. Brown et al. in U.S. Pat. No. 3,213,357 entitled "Earth Formation and Fluid Material Investigation by Nuclear Magnetic Relaxation Rate Determination".
The Schlumberger Nuclear Magnetic Logging Tool measures the free precession of proton nuclear magnetic moments in the earth's magnetic field by applying a relatively strong DC polarizing field to the surrounding rock formation in order to align proton spins approximately perpendicularly to the earth's magnetic field. The polarizing field must be applied for a period roughly five times T1 (the spin-lattice relaxation time) for sufficient polarization (approximately two seconds). At the end of polarization, the field is turned off rapidly. Since the protons spins are unable to follow this sudden change they are left aligned perpendicularly to the earth's magnetic field and precess about this field at the "Larmor Frequency" corresponding to the local earth's magnetic field (roughly from 1300 to 2600 Hz, depending on location).
The spin precession induces in a pick-up coil a sinusoidal signal whose amplitude is proportional to the density of proton present in the formation. The signal decays with a time constant T2* (transverse relaxation time) due to non-homogeneities in the local magnetic field over the sensing volume.
While there have been improvements in the Schlumberger Nuclear Magnetic Logging Tool since its introduction, several of its disadvantages have still not been overcome. For example, the technique using the Schlumberger Nuclear Magnetic Logging tool involves a suppression of very high undesirable signals coming from the bore fluid which is in close proximity to the probe and thus requires doping of the bore fluid with para-magnetic materials. Needless to say this process is both costly and time consuming. Moreover, the Schlumberger nuclear magnetic logging technique is slow in carrying out a spin-lattice relaxation time (T1) measurement thereby limiting commercially operable speeds.
Another technique for non-destructive determination of porosity movable fluid and permeability of rock formation is the so-called "Los Alamos NMR Technique". That technique is described in the following publications: R. K. Cooper and J. A. Jackson, entitled "Remote (Inside-Outside) NMR. I--Production of a Region of Homogenous Magnetic Field," J. Magn. Reson. 41,400 (1980); L. J. Burnett and J. A. Jackson, entitled "Remote (Inside-Outside) NMR. II--"Sensitivity of NMR Detection for External Samples," J. Magn. Reson. 41,406 (1980); J. A. Jackson, L. P. Burnett and J. F. Harmon entitled "Remote (Inside-Outside) NMR. III--"Detection of Nuclear Magnetic Resonance in a Remotely Produced Region of Homogeneous Magnetic Field," J. Magn. Reson. 41,411 (1980); and U.S. Pat. No. 4,350,955 (Jackson et al.) entitled "Magnetic Resonance Apparatus".
The Los Alamos NMR technique is based on the development of a specific magnet/RF coil assembly. Such a structure allows one to obtain the NMR signal predominently from a toroidal or "doughnut" shaped region in the surrounding rock formation at a specified distance from the bore hole axis.
In U.S. Pat. No. 4,350,955 (Jackson et al.) the magnet structure for producing the field comprises a pair of elongated magnets disposed so that their corresponding opposing pole faces are separated by a predetermined distance. Using a magnet structure arranged in that manner results in the production of a field like that shown in FIG. 6 herein. Moreover, as can be seen in the corresponding graph of FIG. 4, the strength of the field produced on the midplane between the poles increases from zero on the axis of the magnets to a maximum, and then decays as the distance from the axis of the magnets increases. Furtherstill, the field is a constant maximum value over a short distance measured radially from the axis of the magnets. Thus, at the location of the maximum flux density a narrow region of uniform field is created, with the field itself being toroidal in shape.
The Los Alamos approach while offering some advantages over the Schlumberger technique, does not eliminate the bore fluid signal problem nor does it overcome the difficulty of unacceptably slow operational speeds due to a low signal-to-noise ratio. Recognizing these problems Jackson proposes to increase significantly the static magnetic field strength but admits that such is impractical with the present state of magnet technology.